FormatJson: minor cleanup

* Prefer feature detection over version comparison.
* Prefer pre- over post- increment and decrement operators.
* Remove the statement that FormatJson::decode() decodes `true`,
  `false`, and `null` case insensitively. Nobody should assume
  this is (or is not) the case, even though the PHP manual says so.
* Avoid using the ternary operator with long strings; prior to
  PHP 5.4, the operator prevented the copy-on-write optimization.
* Avoid placing comments on the same lines as code.

Change-Id: I8fc88e9b7b49aa0cbd4128216557836a3b2cd011
Merge "FormatJson: minor cleanup"